Meiosis Review. Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis,...

Meiosis Review

Meiosis reduces the number of chromosome sets from diploid to haploid

• Like mitosis, meiosis is preceded by the replication of chromosomes

• Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

• The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

• Each daughter cell has only half as many chromosomes as the parent cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Stages of Meiosis

• In the first cell division (meiosis I), homologous chromosomes separate

• Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division

• In the second cell division (meiosis II), sister chromatids separate

• Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division


Fig. 13-7-1Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes







Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes







Meiosis I


1

Haploid cells withreplicated chromosomes

Meiosis II

2 Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

• Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids

• The sister chromatids are genetically identical and joined at the centromere

• The single centrosome replicates, forming two centrosomes

BioFlix: Meiosis


Fig. 13-8

Prophase I Metaphase I Anaphase I Telophase I andCytokinesis

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis

Centrosome(with centriole pair)

Sisterchromatids Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Sister chromatidsremain attached


Cleavagefurrow

Sister chromatidsseparate Haploid daughter cells

forming

• Division in meiosis I occurs in four phases:– Prophase I– Metaphase I– Anaphase I– Telophase I and cytokinesis


Metaphase I

Fig. 13-8a

Prophase I Anaphase I Telophase I andCytokinesis



Spindle




Metaphaseplate




Cleavagefurrow

Prophase I• Prophase I typically occupies more than 90% of the

time required for meiosis• Chromosomes begin to condense• In synapsis, homologous chromosomes loosely pair

up, aligned gene by gene


• In crossing over, nonsister chromatids exchange DNA segments

• Each pair of chromosomes forms a tetrad, a group of four chromatids

• Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred


Metaphase I

• In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole

• Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

• Microtubules from the other pole are attached to the kinetochore of the other chromosome


Fig. 13-8b

Prophase I Metaphase I



Spindle


Metaphaseplate




Anaphase I• In anaphase I, pairs of homologous chromosomes

separate• One chromosome moves toward each pole, guided

by the spindle apparatus• Sister chromatids remain attached at the

centromere and move as one unit toward the pole


Telophase I and Cytokinesis• In the beginning of telophase I, each half of the cell

has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells


• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

• No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated


Fig. 13-8c

Anaphase I Telophase I andCytokinesis



Cleavagefurrow

• Division in meiosis II also occurs in four phases:– Prophase II– Metaphase II– Anaphase II– Telophase II and cytokinesis

• Meiosis II is very similar to mitosis


Fig. 13-8d

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis


forming

Prophase II• In prophase II, a spindle apparatus forms• In late prophase II, chromosomes (each still

composed of two chromatids) move toward the metaphase plate


Prophase II• In prophase II, a spindle apparatus forms• In late prophase II, chromosomes (each still

composed of two chromatids) move toward the metaphase plate


Metaphase II• In metaphase II, the sister chromatids are arranged

at the metaphase plate• Because of crossing over in meiosis I, the two sister

chromatids of each chromosome are no longer genetically identical

• The kinetochores of sister chromatids attach to microtubules extending from opposite poles


Fig. 13-8e

Prophase II Metaphase II

Anaphase II• In anaphase II, the sister chromatids separate• The sister chromatids of each chromosome now

move as two newly individual chromosomes toward opposite poles


Telophase II and Cytokinesis• In telophase II, the chromosomes arrive at opposite

poles• Nuclei form, and the chromosomes begin

decondensing


• Cytokinesis separates the cytoplasm• At the end of meiosis, there are four daughter cells,

each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell


Fig. 13-8f

Anaphase II Telephase II andCytokinesis


forming

Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

• In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs


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Meiosis Review. Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis,...

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